The United States has warned North Korea against any provocative acts after the shock execution of leader Kim Jong-Un's uncle, as the reclusive state campaigned to rally support behind the young supremo.

Washington also sought to step up talks with its Asian allies, voicing concern over regional stability after Jang Song-Thaek - seen as Kim's political regent and the country's unofficial number two - was executed on Thursday after a special military trial.

"Certainly, it's something we're concerned about and we would urge the North Koreans not to take provocative acts ... because it's not in the interest of regional stability," said US State Department spokeswoman Marie Harf.

The comments came after South Korea's defence chief Kim Kwan-Jin pledged to increase military vigilance against any potential provocations, saying the stunning purge indicated Kim Jong-Un's firm resolve to tighten his grip on power.


"This case can be seen as part of the reign of terror by Kim Jong-Un as he is seeking to consolidate his power with an iron fist."

Meanwhile, the reclusive state launched a fresh media blitz in a bid to rationalise Jang's elimination and rally support behind the young leader.

Rodong Sinmun, the official daily, splashed on its front page a colour photo of Kim, wearing a greatcoat with his hands in his pockets, touring a military design institute in his first public activity since the purge. He was accompanied by Choe Ryong-Hae, a confidant who holds the military rank of vice-marshal and trailed by other military officers.

The inspection trip was apparently aimed at displaying Kim's firm grip on power after the shocking elimination of Jang, condemned by Pyongyang as "a traitor for all ages".

Jang, 67, played a key role in cementing the leadership of the inexperienced Kim when he succeeded his father Kim Jong-Il in 2011 but analysts said his power and influence had become increasingly resented. Harf denounced his execution as an incredibly brutal act that underscores North Korea's horrific human rights record but refused to speculate on the reasons behind the purge.

"We're going to increase our discussions with our allies and partners in the region about the internal situation in North Korea," she said.

Washington is in regular talks with Beijing, Pyongyang's main ally and "we're on the same page in terms of urging the North Koreans to come back in line with their international obligations", Harf said.

The regime accused Jang of betraying the trust of both Kim Jong-Un, who is aged around 30, and his father. Jang was also accused of slighting the young leader - not applauding him enthusiastically enough at party meetings.